Lubricating oil compositions, in particular for the automotive industry, make use of a large number of additives, each having its respective role.
Among the most important additives are dispersants, which, as their name indicates, are used to guarantee engine cleanliness and to keep carbonate residues in suspension.
The most widely used dispersants today are products of the reaction of succinic anhydrides substituted in alpha position by an alkyl chain of polyisobutylene (PIBSA) type with a polyalkylene amine, optionally post-treated with a boron derivative, ethylene carbonate or other post-treatment reagents known in the specialized literature.
Among the polyamines used, polyalkylene-amines are preferred, such as diethylene triamine (DETA), triethylene tetramine (TETA), tetraethylene pentamine (TEPA), pentaethylene hexamine (PEHA) and heavier poly-alkylene-amines (HPA).
These polyalkylene amines react with the succinic anhydrides substituted by alkyl groups of polyisobutylene (PIBSA) type to produce, according to the molar ratio of these two reagents, mono-succinimides, bis-succinimides or mixtures of mono- and bis-succinimides
Such reaction products, optionally post-treated, generally have a non-zero basic nitrogen content of the order of 5 to 50, as measured by the total base number or TBN, expressed as mg of KOH per gram of sample, which enables them to protect the metallic parts of an engine while in service from corrosion by acidic components originating from the oxidation of the lubricating oil or the fuel, while keeping the said oxidation products dispersed in the lubricating oil to prevent their agglomeration and their deposition in the casing containing the lubricating oil composition.
These dispersants of mono-succinimide or bis-succinimide type are even more effective if their relative basic nitrogen content is high, i.e. in so far as the number of nitrogen atoms of the polyamine is larger than the number of succinic anhydride groups substituted by a polyisobutenyl group.
However, the higher the basic nitrogen content of these dispersants, the more they favour the attack of the fluorocarbon elastomer seals used in modern engines, because the basic nitrogen tends to react with the acidic hydrogen atoms of this type of seal, and this attack results in the formation of cracks in the elastomer surface and the loss of other physical properties sought in this type of material.
In order to resolve this dilemma, it has been proposed, according to U.S. Pat. No. 5,326,552 filed by the company Chevron, to subject the dispersants of bis-succinimide type to a post-treatment by reaction with a cyclic carbonate. Such a process not only improves the sludge dispersion in a lubricating oil containing these additives, but also the compatibility of the oil with a fluorocarbon elastomer.
Another solution is the subject of a Patent Application WO 93/07242, also filed by Chevron, in which the compatibility of a lubricating oil comprising additives containing basic nitrogen atoms with fluorocarbon elastomers is guaranteed by the addition of borated aromatic polyols, such as borated alkyl catechols.
Furthermore, it is well known that, in order to meet the longevity requirements demanded today in internal combustion engines, the lubricating oil compositions must contain a great number of other ingredients, each of which has a very specific role.
Accordingly, besides the dispersants of the preceding type, other detergents are added, such as sulphonates, alkylphenates or metallic salicylates, sulphurized or not, anti-oxidants, in particular zinc dialkyl dithiophosphates, extreme pressure agents, foam inhibitors, friction reducers, rust removing agents, corrosion inhibitors, pour point depressants, viscosity improvers and many other additives.
Among the additives thus used as agents to reduce friction between moving surfaces in engines are borated glycerol or thio-glycerol esters.